Functional electrical stimulation restores for many patients having implanted devices, as for example, pacemakers, defibrillators, bladder stimulators, implants for coping with pain, tremor, epilepsy and for recovering the sense of hearing, body functions which have been lost. For this, implantable nerve electrodes are applied.
In prior art, it is known to produce implantable nerve electrodes by means of laser treatment from medical silicone and a metal foil. The basis of the known process is the separation of conductive paths and contact areas from a metal foil typically 5 to 25 μm thin by means of a laser. The conductive path, electrode and contact areas are embedded into medical silicone, whereby the single signal paths may be isolated electrically from each other. The contact areas subsequently are exposed by means of the laser.
This known technology, however, has two problems: The conductive paths are extremely fragile due to their fineness. The elastic silicone may only protect them to a very limited extend from mechanical influences, as, for example, while handling during the implantation. Accordingly, laser processed nerve electrodes, on the one hand, are relatively prone to breakage of conductive paths. On the other hand, silicone has a low electric strength. When a stimulation is carried out by means of nerve electrodes electrically, voltages of several 10V between two adjacent conductive paths may occur. According to manufacturer specifications, the electric strength of silicone may be lowered to 2 kV/mm by storing it in water. A voltage of 20 V between two adjacent conductive paths with a distance of 10 μm, thus, may lead to an electrical break down. This fact limits the integration density of the electrode.
In order to solve the problem of the inadequate stability, it is known in prior art to, for example, arrange the conductive paths in meandering lines. Hereby, a certain extensibility of the conductive paths is achieved. However, the increased space requirement is disadvantageous for the meandering arrangement of conductive paths, which in turn has a negative impact on the maximum integration density.
A further approach involves the embedding of conductive paths in thicker and harder silicone. However, also this variant has little success, because also thicker silicone is much more elastic than the metal embedded therein. The occurring mechanical forces further influence the conductive paths substantially, and lead to their damage.
It also has been attempted to increase the mechanical stability by adding a polymer foil which is mechanically very strong and a further silicone layer, which have been inserted into the multi-layer structure of silicone-metal-silicone. Hereby, an improvement of the mechanical stability of the nerve electrode may, however, be achieved, but the new multi-layer structure silicone-metal-silicone-polymer-foil-silicone is unfavorable in that the layer of the rigid polymer foil which is non-compressible or non-extendable defines the mechanically neutral fiber within the multi-layer structure. With strong bending movements, therefore, compressional or tensile forces of the metal conductive paths occur.
With respect to the second problem of the low electrical strength of the silicone in which the conductive paths are embedded, up to now no approach to a solution has been found.
Therefore, it is an object of the present invention to provide an implantable nerve electrode and a method for producing a nerve electrode according to which an effective protection for the conductive paths and at the same time a high integration density may be achieved.